1. Field of the Invention
My invention involves generally methods for radio frequency (RF), electromagnetic interference (EMI) and electrostatic shielding and, more specifically, involves methods for blocking electrostatic fields without blocking RF magnetic fields.
2. Description of the Related Art
Electrostatic discharge is defined in the U.S. Military Handbook DOD-HKBK-263 as a "transfer of electrostatic charges between bodies at different potentials caused by direct contact or induced by an electrostatic field." This transfer of electrostatic charge causes destruction to electronic components, which are increasingly susceptible to such destruction. Electrostatic discharge (ESD) is the primary source of damage to and failures of semiconductor devices today.
Most ESD related efforts have been by practitioners in the microcircuit manufacturing arts and involve the prevention of electrostatic charge accumulation by a variety of means. Relatively little effort goes into hardening the modern electronic microcircuit device against the effects of ESD. Considerable confusion and misunderstanding of electrostatic electricity exist in the microcircuit manufacturing industry because of this prevailing emphasis on electrostatic control products. Most electrostatic control products operate to reduce or eliminate the accumulation of electrostatic charges through grounding or ionization. Personnel-generated electrostatic charge is commonly controlled with a wrist strap that works as intended but does not completely control the problem because it protects only at the workstation and does not remove accumulated electrostatic charge from clothing. Personnel approaching a workstation can carry potentially damaging electrostatic charges until a wrist strap is attached.
The accumulation of static electricity occurs freely in nature. In factories with nonconductive floors and no means of humidity control, a worker merely walking across the floor can accumulate an electrostatic charge because of the triboelectric effect known in the art. With humidity under 20%, the triboelectric effect causes electrostatic charge accumulation merely from the repeated making and breaking of contact between a worker's shoes and the floor.
As a worker approaches a static-sensitive device at a workstation, the worker's accumulated electrostatic field reaches the device ahead of the worker and induces a current sufficient to damage or destroy. As the worker arrives at the workstation, the electrostatic field is now close enough to polarize the sensitive devices so that damage can again result without the devices having been touched. Then, if the worker reaches out to pick up a device, an electrostatic discharge (ESD) jumps from fingertip to device, creating an electromagnetic pulse (EMP) that can damage all nearby devices by inducing destructive currents. Practitioners in the art believe that as much as 25% of all electronic component failures are caused by ESD.
Because electrostatic charge on one body can be imparted to another body through induction and conduction, ESD damage can result from unsuspected sources. Electron transfer will occur when two contacting bodies are rubbed together, separated, or flow past one another. These physical facts demonstrate that the accumulation of electrostatic charges cannot be prevented or eliminated but only controlled. Moreover, because electrostatic charges and discharges occur everywhere, the effects of such charges on sensitive electronic components cannot be eliminated through elimination of the charge itself, but must be prevented through shielding of devices from the damaging effects of accumulated electrostatic charges.
Common methods for protecting sensitive electronic components from the effects of accumulated electrostatic charges include the use of antistatic and conductive packaging materials. Antistatic packaging materials are produced by adding an organic antistatic additive to a plastic base material to form a hygroscopic plastic. Antistatic materials do not act as shields but instead function by forming a conductive moisture layer that minimizes the accumulation of electrostatic charge and tends to equalize the charge over the entire package surface. Conductive materials, while not retarding electrostatic charge accumulation, do provide shielding and also equalize the charge over the packaging surface and allow rapid charge dissipation when grounded. Conductive packaging materials can be produced by depositing continuous metal layers onto a base layer or coating a base layer with conductive paints or carbon-loaded conductive ink.
Antistatic bags using polyethylene sheet material folded at the bottom and sealed on the side are widely used as containers for sensitive components. Such a bag comprises a humidity-independent conductive outer layer, a middle insulation layer, and a humidity-dependent antistatic inner layer and can be transparent to permit examination of components without removal from the bag. Variations on this antistatic bag concept include bags comprising layers of aluminum or nickel foil, carbon-loaded plastic, and other similar materials known in the art.
Conductive boxes and enclosures using materials similar to those employed in these bags will also protect sensitive electronic components from the effects of accumulated electrostatic charges. An outer conductive metal or carbon-loaded plastic layer can serve as an excellent Faraday shield, which shields the contents from both electrostatic charges and electromagnetic radiation or pulses.
Practitioners in the art have responded to the need for electrostatic shielding by developing a variety of means for shielding sensitive components. For instance, U.S. Pat. No. 4,542,076, issued to Jurgen Bednarz, et al., on Sep. 17, 1985, discloses a method for metallizing plastic by roughening the plastic surface, plating the surface with metal, and adding an outer coating to hold the metal in place. Bednarz's method is suitable for metallizing plastic shrink wrap for use in shielding cable connectors and the like. The metallized plastic is nonpermeable to electromagnetic radiation.
U.S. Pat. No. 4,663,240, issued to Juan Hajdu, et al., on May 5, 1987, discloses a method for coating plastic enclosures with two layers of a special metallized paint to form a 40 dB radio frequency (RF) electromagnetic shield. Hajdu does not consider the problem of electrostatic shielding.
U.S. Pat. No. 4,851,608, issued to Donald P. Seip, on Jul. 25, 1989, discloses a method for manufacturing honeycomb shields for covering enclosure openings for cooling fans and the like. Seip addresses the problem of air-permeable electromagnetic shields but does not consider the electrostatic shielding problem. Similarly, U.S. Pat. No. 4,262,365, issued to John B. George, on Apr. 14, 1981, discloses a thin metal RF electromagnetic shield with slots and louvers disposed to transmit and scatter infrared radiation (IR) while shielding against electromagnetic radiation. George's IR permeable shield does not relate to the electrostatic shielding problem.
U.S. Pat. No. 3,479,622, issued to W. Freer, on Nov. 18, 1969, discloses a radio frequency tuner having two sections separated by an iris window with a plurality of conductive fingers extending across the window. This window and finger arrangement acts as a high-pass filter that blocks low frequency electrostatic fields while remaining transparent to high frequency electromagnetic fields. Freer neither teaches nor suggests the application of his low pass filter technique to the protection of sensitive electronic components.
None of these techniques are useful for the protection of portable electronic devices such as calculators, smart cards and radios, which require protection from accumulated electrostatic charges and ESDs that occur when a charged user handles the device or contacts a nearby conductive surface. The shielded bag and box used in microcircuit manufacturing, while providing good protection from ESD, also block the transmission of electromagnetic energy necessary for the proper operation of many portable electronic devices. The typical wrist strap and antistatic container techniques are practical only during a manufacturing or assembly process. The casual user of a portable electronic device will have neither wrist strap grounding nor special antistatic shoes.
When exposed to ESD, a typical electronic device experiences rapid changes in capacitively-coupled voltage at the input of the microcircuit components, often sufficient to cause device malfunction if not destruction. The best method for shielding against these effects is the use of a grounded conductive metal surface as is known in the art. A plastic surface filled with conductive particles is less effective because capacitive-coupling of the rapid ESD transients is still possible. Thus, the only effective electrostatic shielding means known in the art are those metal surfaces that also block the transmission of electromagnetic radiation.
There is a strongly-felt need in the art for an electrostatic shield that protects against ESD and can be used with portable electronic devices such as calculators, smart cards and radios without preventing the transmission of the RF electromagnetic fields required for proper function. This need is acute for devices that use radio frequencies above 15 MHz because of the bandwidth requirements of modern micro-technology. These unresolved problems and deficiencies are clearly felt in the art and are solved by my invention in the manner described below.